SPMs are semiconductor photon sensitive devices made up of an array of very small Geiger-mode avalanche photodiode (APD) cells on a substrate such as silicon. An example 10χ10 microcell array is shown in FIG. 1 of the accompanying drawings. Each cell is connected to one another to form one larger device with one signal output. The entire device size may be as small as 1χ1 mm or much larger. FIG. 2 of the accompanying drawings is a schematic diagram of a conventional silicon photomultiplier.
APD cells vary in dimension from 20 to 100 microns depending on the mask used, and can have a density of up to 1000/sq. mm. Avalanche diodes can also be made from other semiconductors besides silicon, depending on the properties that are desirable. Silicon detects in the visible and near infrared range, with low multiplication noise (excess noise). Germanium (Ge) detects infrared to 1.7 μm wavelength, but has high multiplication noise. InGaAs (Indium Gallium Arsenide) detects to a maximum wavelength of 1.6 μm, and has less multiplication noise than Ge. InGaAs is generally used for the multiplication region of a heterostructure diode, is compatible with high speed telecommunications using optical fibres, and can reach speeds of greater than Gbit/s. Gallium nitride operates with UV light. HgCdTe (Mercury Cadmium Telluride) operates in the infrared, to a maximum wavelength of about 14 μm, requires cooling to reduce dark currents, and can achieve a very low level of excess noise.
Silicon avalanche diodes can function with breakdown voltages of 100 to 2000V, typically. APDs exhibit internal current gain effect of about 100-1000 due to impact ionization, or avalanche effect, when a high reverse bias voltage is applied (approximately 100-200 V in silicon). Greater voltage can be applied to silicon APDs, which are more sensitive compared to other semiconductor photodiodes, than to traditional APDs before achieving breakdown allowing for a larger operating gain, preferably over 1000, because silicon APDs provide for alternative doping. Reverse voltage is proportional to gain, and APD gain also varies dependently on both reverse bias and temperature, which is why reverse voltage should be controlled in order to preserve stable gain. SPMs can achieve a gain of 105 to 106 by using Geiger mode APDs which operate with a reverse voltage that is greater than the breakdown voltage, and by maintaining the dark count event rate at a sufficiently low level.
Geiger-mode APDs produce relatively large charge pulse when struck by a photon of the same amplitude no matter the energy of the photon. When reading out conventional APDs, noise of the preamplifier significantly degrades timing and amplitude resolution performance for short (shorter than approximately 500 ns) light pulses. Compared to conventional APDs, SPMs using Geiger mode APDs provide much higher output amplitude, essentially eliminating the impact of preamplifier noise.
Many SPM [Silicon Photomultiplier] applications call for a fast light-to-current response, with order of 1 ns or even shorter time constants. Improved time response would benefit such applications as time-resolved spectroscopy, LIDARs, TOF [time of flight] PET [Positron Emission Tomography] etc.
At the moment use of the SPM for ‘fast’ applications, especially large area SPMs is seriously compromised by the fact that the bulk of the SPM's signal charge is released as an exponentially decaying current with a long ˜50 ns time constant. However, the avalanche development process in the SPM APD is extremely fast and the long time constant arises due to the fact that the APD signals are read out through the distributed SPM's biasing circuitry.
It is desirable to provide new electrode detector configurations to enhance currently known Silicon Photomultiplier [SiPM], also known as SPM, MicroPixel Photon Counters [MPPC], MicroPixel Avalanche Photodiodes [MAPD] with improved performance in such areas as Positron Emission Tomography [PET], including Time-Of-Flight PET [TOF-PET], Laser Ranging [LIDAR] applications, bio luminescence, High Energy Physics [HEP] detectors.
Currently known Silicon Photomultipliers provide minimum output rise time in the order of 1 nS and fall time of at least 10 ns. This is much longer than conventional vacuum Photomultiplier (PMTs) or silicon avalanche photodiodes (APDs) resulting in performance loss and complication of readout electronics. SPMs in attempt to address this problem have been designed to include a fast terminal readout terminal. However, due to the fast terminal's AC coupled nature, the output includes an undesirable overshoot during microcell recovery.
There is therefore a need to provide a semiconductor photomultiplier which addresses at least some of the drawbacks of the prior art.